1. Field of the Invention
This invention relates generally to a system and method for controlling the relative humidity of the cathode exhaust gas from a fuel cell stack and, more particularly, to a system and method for controlling the relative humidity of the cathode exhaust gas from a fuel cell stack that includes changing the operating range of the cathode pressure based on stack current density during stack power transients.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Excessive stack temperatures may damage the membranes and other materials in the stack. Fuel cell systems therefore employ a thermal sub-system to control the temperature of the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling fluid flow channels in the bipolar plates in the stack to draw away stack waste heat. During normal fuel cell stack operation, the speed of the pump is controlled based on the stack load, the ambient temperature and other factors, so that the operating temperature of the stack is maintained at an optimal temperature, for example 80° C. A radiator is typically provided in a coolant loop outside of the stack that cools the cooling fluid heated by the stack where the cooled cooling fluid is cycled back through the stack.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack is controlled to control the relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack. For stack durability purposes, it is desirable to minimize the number of relative humidity cycles of the membrane because cycling between RH extremes has been shown to severely limit membrane life. Membrane RH cycling causes the membrane to expand and contract as a result of the absorption of water and subsequent drying. This expansion and contraction of the membrane causes pin holes in the membrane, which create hydrogen and oxygen cross-over through the membrane creating hot spots that further increase the size of the hole in the membrane, thus reducing its life. Further, the fuel cells would be less prone to flooding if the cathode outlet RH is less than 100%. Also, by reducing liquid water in the stack, the stack can be more easily purged at shut-down to reduce the chance of freezing.
During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, the water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, droplets form in the flow channels. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds. As the droplet size increases, surface tension of the droplet may become stronger than the delta pressure trying to push the droplets to the exhaust manifold so the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow.
The relative humidity (RH) of the cathode outlet gas is a function of the cathode stoichiometry, the pressure of the cathode outlet gas and the temperature of the cooling fluid exiting the stack. From an RH control perspective it is desirable to maintain the cathode stoichiometry, the cathode outlet gas pressure and the cathode outlet gas temperature substantially constant to maintain the desired relative humidity. However, there are certain limitations and realities that the fuel cell system must meet in order to provide efficient and effective performance.
A fuel cell system controller will typically use a cathode outlet gas pressure table that identifies a certain cathode outlet pressure depending on the current density being generated by the stack. Because stack voltage increases with pressure, a higher cathode pressure is generally provided as the stack current density increases. Further, high cathode pressures at idle would cause significant compressor parasitics. For a low stack current density, the lower end of the cathode pressure range may be about 102 kPa and for a high stack current density, the cathode pressure may be about 143 kPa. The cathode stoichiometry at low current density may be around 5 to force water out of the cathode flow channels to provide voltage stability. The cathode stoichiometry at high current density may be around 1.8 because of the limitations on the compressor speed.